This invention relates to acceleration sensors micromachined from silicon and, more particularly, sensors having an inertial mass positioned by torsional or cantilever support members.
It is known in the art that small compact acceleration sensors may be formed by micromachining silicon wafers into suitable configurations that are capable of detecting acceleration along one axis. The micromachining process is normally performed on batches of silicon wafers. This process consists of masking and forming patterns of etch stop material on a wafer surface, etching the exposed silicon, removing the etch stop material, metallizing, and bonding. The silicon wafers are diced into individual acceleration sensor devices which are packaged and connected to suitable electronic circuitry to form accelerometers. Using these techniques, a two axis or three axis acceleration sensor requires two or three discrete diced devices, respectively, to be precisely mechanically aligned along two or three orthogonal axes of acceleration. Examples of acceleration sensors formed by a micromachining process are described in the following U.S. Pat. Nos. 4,574,327; 4,930,043; and 5,008,774.
Prior forms of silicon acceleration sensors employ an inertial mass which moves in response to acceleration, positioned by cantilever support members that may introduce an asymmetry that can result in an undesirable cross-axis sensitivity. To avoid this undesirable asymmetric effect, these devices are designed with flexible support members around the periphery of an inertial mass so that the response to acceleration is preferentially along an axis perpendicular to the plane of the inertial mass and the support members. To further limit the acceleration response to one axis, the support members are sometimes placed in the mid-plane of the inertial mass or symmetrically placed at the top and bottom surfaces of the inertial mass. The devices fabricated in this manner may exhibit wide parameter variations between devices. Furthermore, for multiple axes applications, multiple discrete devices must be precisely aligned mechanically to each axis of acceleration. Difficulties encountered in the fabrication include the accurate location of the mid-plane and precise alignment of multiple devices, making the fabrication process complex, slow and expensive.
For the foregoing reasons, there is a need for a monolithic multiple axes acceleration sensor micromachined from silicon by a relatively simple fabrication process that results in low mechanical stress, temperature stable devices with tight parameter tolerances between devices. It is desirable that any required multiple axes alignment be performed as a part of the lithographic process used in the device fabrication rather than require precise mechanical alignment of discrete devices after the dicing operation. It is further desirable that the fabrication process be adjustable on a batch basis, in order to produce devices with predetermined acceleration sensitivity, with batches ranging from low sensitivity devices to high sensitivity devices.
The present invention is directed to a low mechanical stress, temperature stable, monolithic multiple axes acceleration sensor with tight parameter tolerances between devices that is micromachined from silicon by a relatively simple fabrication process. Because the present monolithic multiple axes acceleration sensor may be aligned by the lithographic process used in device fabrication, the need for precise mechanical alignment of discrete sensor devices along orthogonal axes of acceleration is eliminated. The fabrication process of the present invention may be adjustable on a batch basis, in order to produce devices with predetermined acceleration sensitivity, with batches ranging from low sensitivity devices to high sensitivity devices.
Where prior forms of silicon acceleration sensors attempted to avoid asymmetric cross-axis sensitivity, the present invention exploits this cross-axis effect to enable fabrication of a monolithic multiple axes acceleration sensor. The present silicon acceleration sensor invention comprises one, two, three or four silicon acceleration sensor cells, where each sensor cell comprises a movable silicon inertial mass that moves in response to acceleration and is positioned by beam members coplanar with a first surface of the silicon inertial mass and fixed to a silicon support structure. A means is provided for detecting movement of the inertial mass or resulting flexure of the beam members due to acceleration of the inertial mass and the silicon support structure. The relative position of each inertial mass is at right angles to an adjacent inertial mass when viewing the first surface of each silicon mass, using the position of the beam members as angular reference. A silicon acceleration sensor device embodying the present invention having a single sensor cell comprising one movable silicon inertial mass can sense acceleration in two orthogonal axes but cannot distinguish between acceleration along one axis or the other. A device having two sensor cells, where each sensor cell comprises a movable silicon inertial mass positioned at a 180 degree angle to the inertial mass of the other sensor cell when viewing the first surfaces of the inertial masses using the beam members as an angular reference, can sense acceleration in two orthogonal axes and can distinguish between acceleration along both axes. A device having three sensor cells, where each sensor cell comprises a movable silicon inertial mass positioned at angles of 0, 90, and 180 degrees relative to each other when viewing the first surfaces of the inertial masses using the beam members as an angular reference, can sense acceleration along three orthogonal axes and can distinguish between acceleration along each of the three axes. A device having four sensor cells, where each sensor cell comprises a movable silicon inertial mass positioned at angles of 0, 90, 180, and 270 degrees relative to each other when viewing the first surfaces of the inertial masses using the beam members as an angular reference, can sense acceleration along three orthogonal axes and can distinguish between acceleration along each of the three axes. The device comprising four sensor cells is of a physically symmetrical geometry when viewing the first surface of each inertial mass, and provides the capability for cancellation of opposing direction non-linearities. Thus, multiple axes acceleration sensing is achievable with a single monolithic device that does not require precise mechanical alignment of multiple discrete single axis acceleration sensing devices. One means for detecting movement of the inertial mass is by measuring the capacitance between the first surface of the movable inertial mass and a first electrically conductive layer spaced from the first surface and fixed in reference to the supporting silicon structure; and by measuring the capacitance between a second surface of the movable inertial mass opposite the first surface and a second electrically conductive layer spaced from the second surface and fixed in reference to the supporting silicon structure. Another means for detecting movement of the inertial masses is by measuring the resistance of piezoresistive elements placed on the positioning beam members. The beam members may be either in a cantilever or torsion configuration. The shape of the inertial mass is generally described as being a rectangular parallelepiped in the preferred embodiment of the invention.
A method of manufacture of a silicon acceleration sensor device, having a single silicon acceleration sensor cell with an electrically conductive silicon movable inertial mass, comprises the forming of a layered sandwich of an etch-stop layer between a third wafer section of electrically conductive silicon and a fourth wafer section of electrically conductive silicon, the third wafer section of silicon having an exposed first surface and the fourth wafer section of silicon having an exposed second surface. A second section of the silicon inertial mass is formed by etching a rectangular frame-shaped channel in the fourth wafer section from the exposed second surface extending to the etch-stop layer. A first section of the silicon inertial mass is formed by etching a U-shaped channel and a bar-shaped channel in the third wafer section from the exposed first surface extending to the etch-stop layer, positioning the bar-shaped channel and the U-shaped channel in the third wafer section to be in horizontal alignment with, and of equal planar dimensions to the rectangular frame-shaped channel in the fourth wafer section. Means are provided to electrically connect the second section of the inertial mass to the first section of the inertial mass through the etch-stop layer or on the etched surface of the inertial mass. The silicon dioxide layer that is exposed by the etched frame-shaped channel, the etched U-shaped channel, and the etched bar-shaped channel is then stripped away, thereby creating a rectangular parallelepiped-shaped movable silicon inertial mass positioned by beam members fixed to a silicon support structure. An alternative means of electrically connecting the second section of the inertial mass to the first section of the inertial mass is to deposit a layer of conductive polysilicon over the resulting etched and stripped structure. This deposition process could also be used where it is desired to use nonconductive silicon wafer sections. Means are provided to detect movement of the silicon inertial mass by fixing a first electrically conductive layer, spaced from the first surface of the inertial mass, relative to the silicon support structure for a first capacitance measurement between the first surface of the inertial mass and the first electrically conductive layer; and by fixing a second electrically conductive layer, spaced from the second surface of the inertial mass, relative to the silicon support structure for a second capacitance measurement between the second surface of the inertial mass and the second electrically conductive layer. These electrically conductive layers are preferably metallic in composition. Alternatively, means for detecting movement of the inertial mass may be provided by placing piezoresistive elements on the positioning beam members fixed to the silicon support structure, and measuring the change in resistance when the beam members are flexed or twisted.
In the preferred embodiment of a method of manufacture of a silicon acceleration sensor having at least one silicon acceleration sensor cell, the first section of the movable silicon inertial mass is formed by etching a U-shaped channel and a bar-shaped channel in the first layer of silicon from the exposed first surface extending to the silicon dioxide layer, positioning the bar-shaped channel and the U-shaped channel in the first layer of silicon to be in horizontal alignment with, and of equal planar dimensions to the rectangular frame-shaped channel in the second layer of silicon. The bar-shaped channel is positioned across the open top of the U-shaped channel, centered within the outside dimensions of the open top of the U-shaped channel, and extending in length to equal the entire outside width of the top of the U-shaped channel. The ends of the bar-shaped channel are spatially separated from the top of the U-shaped channel, so that the spatial separation results in a device with a silicon inertial mass positioned by torsion beam members.
Although the preferred embodiment of the invention uses a silicon dioxide layer as an etch-stop layer, there are alternative embodiments. These alternative embodiments include a layer of silicon nitride, a layer of doped silicon, and the depletion layer associated with the junction of two differently doped silicon sections.
In alternative embodiments of a method of manufacture of a silicon acceleration sensor having at least one silicon acceleration sensor cell, the first section of the movable silicon inertial mass is formed by etching a U-shaped channel and a bar-shaped channel in the first layer of silicon from the exposed first surface extending to the silicon dioxide layer, positioning the bar-shaped channel and the U-shaped channel in the first layer of silicon to be in horizontal alignment with, and of equal planar dimensions to the rectangular frame-shaped channel in the second layer of silicon. The bar-shaped channel is positioned across the open top of the U-shaped channel, centered within the inside dimension of the open top of the U-shaped channel, and extending in length to be less than the inside width of the top of the U-shaped channel. The ends of the bar-shaped channel are spatially separated from the inside top of the U-shaped channel, so that the spatial separation results in a device with a silicon inertial mass positioned by cantilever beam members.
A further embodiment of a method of manufacture of a silicon acceleration sensor having at least one silicon acceleration sensor cell is to vary the acceleration sensitivity by adjusting the thickness of the beam members by adjusting the thickness of the first silicon wafer section, by adjusting the width of the beam members by adjusting the spatial separation between the U-shaped channel and the bar-shaped channel, or by adjusting the length of the beam members by adjusting the width of the etched channels.
The method of manufacture of a device having two silicon acceleration sensor cells each comprising a movable silicon inertial mass is identical to the method of manufacture of a device having one silicon acceleration sensor cell comprising one movable silicon inertial mass, except that a second inertial mass is positioned lithographically and then physically at a 90, 180 or 270 (which is the same functionally as 90) degree angle to the first inertial mass when viewing the exposed first surface of the silicon masses, using the positioning beams as angular reference. The method of manufacture of a device having three silicon acceleration sensor cells, each sensor cell comprising a movable silicon inertial mass, is identical to the method of manufacture of a device having two silicon acceleration sensor cells, except that the third inertial mass is positioned lithographically and then physically at a 90 degree angle to the second inertial mass and at a 180 degree angle to the first inertial mass when viewing the exposed first surface of the silicon masses, using the positioning beams as angular reference. The method of manufacture of a device having four silicon acceleration sensor cells, each sensor cell having a movable silicon inertial mass, is identical to the method of manufacture of a device having three acceleration sensor cells, except that a fourth inertial mass is positioned lithographically and then physically at a 90 degree angle to the third inertial mass, at a 180 degree angle to the second inertial mass, and at a 270 degree angle to the first inertial mass when viewing the exposed first surface of the silicon masses, using the positioning beams as angular reference. In this manner, a monolithic multiple axes acceleration sensor is formed that does not require precise mechanical alignment of multiple discrete single-axis acceleration sensors.